In certain types of internal combustion engines, for example in internal combustion engines wherein the cylinders are disposed in line, it is a common practice to utilize a single casting for the inlet and exhaust manifolds or, alternatively where separate castings are used for the inlet and exhaust manifolds, to dispose the manifolds in close proximity with each other such that heat from the exhaust manifold is convectively transferred through the engine cylinder block to the walls of the inlet manifold, thus warming the air-fuel mixture flowing through the inlet manifold and aiding in vaporizing the droplets of fuel entrained by the air flow through the carburetor throat.
In internal combustion engines of the V-type, such as V-8 or V-6 engines, the inlet manifold is generally bolted on the top of the cylinder block between the two inclined banks of cylinders, and a pair of exhaust manifolds are used, each exhaust manifold being bolted over the exhaust ports on the side of each bank of cylinders. Due to the distance separating the exhaust manifolds from the inlet manifold, heat transfer from the exhaust manifolds to the inlet manifold is not readily effected by convection through the mass of the cylinder block and separate heads, with the result that the engine is subjected to long warming up periods, especially in winter, during which fuel remains suspended in the flow of induction air in the form of relatively large droplets and tends to condense on the cold walls of the inlet manifold, resulting in imperfect combustion within the combustion chambers caused by excessive choking of the carburetor inlet to provide an over-rich mixture, with the accompanying symptoms of loss of power, poor acceleration, incomplete combustion and raw fuel being present in the exhaust gases.